I. Field of the Invention
This invention relates to drilling heads for oil and gas wells and, in particular, to a drilling head which employs dual rotating stripper rubbers to seal against the well string preventing the flow of fluids upwardly through the drilling head.
II. Description of the Prior Art
Early drilling heads employed a single stripper rubber to divert the flow of drilling fluid away from the rig floor. The stripper rubber was fixedly mounted within the drilling head and the drill string rotated and moved longitudinally through the stripper rubber as it is attempted to seal against the string. It was determined that the action of the drill string caused considerable wear on the stripper rubber requiring frequent replacement. To reduce the abrasive wear, the stripper rubber was rotated with the drill string to maintain sealing contact. However, a drill string typically includes various diameter sections. For example, the drill collars joining sections of drill string have a greater diameter than the drill pipe itself. Thus, the stripper rubber was sized to maintain sealing contact with the drill pipe or the smallest diameter component which travelled through the drilling head. Because of the different diameters of the drill string, the stripper rubber needed to be rigid enough to withstand the pressures of the drilling fluid yet resilient enough to maintain a seal on the drill collars as they passed through the drilling head and thereafter return to the original configuration to seal against the smaller diameter drill pipe. The operating cycle of the drilling head was directly proportional to number of drill collars which passed through the single stripper rubber since the stripper would not return to its original sealing diameter.
In an attempt to maintain sealing contact with both the smaller drill pipe and the drill collars, dual stripper rubber drilling heads included stripper rubbers with different inner diameters designed to seal against the drill collars and drill pipe. Early versions only rotated one of the stripper rubbers since the lower stripper was fixedly mounted within the drilling head. More recently, dual rotating stripper rubbers, each with a different inner diameter, have been employed in drilling heads by mounting the lower stripper rubber to a drive ring extension which is connected to the drive bushing at the upper end of the drilling head. Despite the advantages of the prior known dual stripper rubber drilling heads, only one of the stripper rubbers is typically in sealing contact with the drill string. Accordingly, the dual stripper rubbers do not provide additional sealing against the flow of drilling fluids.